1. Field of the Invention
The present invention relates to an apparatus and method for measuring signal quality, and more particularly, to an apparatus and method for measuring signal quality that detects the quality of a signal reproduced from an optical disc according to level information of a viterbi decoder.
2. Description of the Related Art
An optical disc drive writes binary signals on a disc surface and reproduces data by radiating a laser beam on the disc surface and analyzing the reflected waveform. The signal read from the disc surface is a radio frequency (RF) signal. Further, even when binary signals are recorded on the disc surface, the RF signal read from the disc surface has features of an analog signal, not a binary signal, due to disc characteristics and optical characteristics.
The features of the analog signal can be used for various purposes, such as quality control of a manufactured disc. Representative features of the analog signal are an asymmetry value and a modulation ratio. The asymmetry value, which is an indicator representing how far the center portion of a signal is located from a total signal, is typically defined as shown in the following Equation 1. The modulation ratio, which is an indicator representing how far an amplitude of a signal with a short time period (T) is apart from an amplitude of a total signal, is typically defined as shown in the following Equation 2.asymmetry value={(longest T top level+longest T bottom level)/2−(shortest T top level−shortest T bottom level)/2}/(longest T top level−longest T bottom level)  Equation 1modulation ratio=(shortest T top level−shortest T bottom level)/(longest T top level−longest T bottom level)  Equation 2
FIG. 1 is a block diagram of a conventional signal quality measuring apparatus for obtaining the asymmetry value and the modulation ratio. The conventional technology is disclosed in U.S. Pat. No. 5,490,127.
Referring to FIG. 1, after an RF signal read from a disc is transmitted through a slicer circuit 41 and 42, a sign (+ or −) of the RF signal is determined, and after the sign determined RF signal goes through a specific period (positive or negative) edge detection circuit 43a and 43b and sample pulse generators 44a and 44b, shortest T and longest T sample pulses are obtained. FIG. 1 shows a sample waveform of a signal in which the shortest T is 3 seconds. After the sample pulses go through a respective sample and hold circuit 46a and 46b and a peak bottom hold circuit 48a and 48b, peak values 3TTOP and RFTOP and bottom values 3TBTM and RFBTM are obtained.
The asymmetry value and modulation ratio are calculated by passing the peak values 3TTOP and RFTOP and the bottom values 3TBTM and RFBTM through an equivalent circuit (not shown) to calculate Equations 1 and 2.
However, since the conventional signal quality detecting apparatus described above detects peak values and bottom values by sampling only specific sample waveforms of RF signals read from a disc, the peak values and bottom values cannot be precisely calculated.
Also, when a shortest T is changed, that is, though 2T is used as the shortest T in a disc writing method at first, when 3T is used instead of 2T in the disc writing method after it is determined that the asymmetry value and modulation ratio cannot be detected using 2T, a hardware configuration 51 through 58 shown in FIG. 1 must be reconfigured, and the reconfiguration is difficult and time consuming.